The present invention relates to a device for conveying material in accordance with the preamble to appended Claim 1.
In many contexts, there is a need to displace material, for example bulk material, and, in such instance, not only homogeneous material but also material which includes components of different sizes, densities, elasticities, moisture content etc. Examples of such materials are grain, refuse, wood chips, coal, coke, dewatered sludge, pellets, flue ash, pulverulent material, paper pulp, fibre materials, etc. In many practical applications, it also applies that such displacement take place from a lower level to a higher level. In this instance, for reasons of space it is often important that such displacement take place substantially vertically or in a steeply inclining path.
Conventional screw conveyors provided with a mechanical shaft and which are utilised as vertical or steeply inclining conveyors suffer from a number of drawbacks. They have a low degree of efficiency and must, therefore, operate at high speeds, normally exceeding 100 rpm, and often up to 300 rpm. This high speed results in considerable energy consumption and often leads to rapid wear. The rigid, fixedly journalled construction and the limited space between threads, shaft and the wall of the casing all entail that material may readily become stuck and that the conveyor has relatively slight throughput capacity. The rotary shaft renders impossible the transport of material which may become wrapped around the shaft. Damp, tacky material readily dries on the inner wall of the casing and constantly reduces the free space between the screw and its casing. Thus, it is well known in the art that the rotation of the screw may be impeded or prevented by this phenomenon.
A spiral or helix provided with a shaft affords no homogeneous filling, which gives rise to vibrations. Sludge and similar material are often not conveyed in a uniform flow but inca random intermittent flow. The unhomogeneous filling and high speed of rotation of the spiral give rise to vibrations when the material is thrown against the walls of the conveyor. This results in severe wear on linings and operational disruption.
The above-outlined drawbacks in vertical screw conveyors provided with a shaft have the effect that conveyors for the displacement of material between different levels are built with relatively slight sloping angle, which naturally implies greater space requirements.
A device according to our European patent No. 0 314 752 provides a satisfactory solution to above-outlined problems and consists of a first, substantially horizontal conveyor consisting of a spiral or helix in a surrounding casing which conveys material to a substantially vertical conveyor, similarly consisting of a spiral in a surrounding casing. In order to increase the capacity of the substantially vertical conveyor section, two parallel spirals are disposed in the vertical casing according to the present invention.
Double spirals give a number of advantages compared with a single spiral. In order to obtain the same conveyor capacity with a single spiral as with double spirals, the single spiral must either be made considerably larger or the speed of rotation must be increased. Increased speed of rotation results in the above-outlined problems in respect of wear and energy consumption. A larger spiral will be considerably heavier which in turn results in additional problems such as increased loading on bearings, the need for larger drive motors, gear wheels etc. and also greatly increased energy consumption. It is, moreover, easier to obtain a symmetric infeed in double-spirals than if only a single spiral is employed. The double spirals support the infeed and discharge in that the spirals rotate in different directions and that the inlet and outlets, respectively, are disposed on opposite sides. This entails that the rotation of the spirals has an induction action on the material at the inlet end and an expulsion action at the outlet end. In those embodiments which have no wall between the spirals, the risk of blockage is moreover reduced and conveyance of individual larger pieces of material is made possible.
It generally applies that, in a screw conveyor with a mechanical shaft, or in a screw conveyor without mechanical shaft, conveyance takes place in that the conveyed material abuts against and slides along the drive surface of a thread which makes an oblique angle with the direction of transport. If this relative displacement between the material and the drive surface does not take place, i.e. if the material adheres to the thread, no material displacement whatever will take place in a direction towards the discharge end of the conveyor, but material instead rotates around with the thread in a circular movement. In order for a displacement towards the discharge end of the conveyor to be able to take place, it is thus necessary that the displacement of the material in the circumferential direction of the thread is arrested such that the thread, on its rotation, displaces the material towards the discharge opening. The frictional forces between the conveyed material and the rotating thread should, in other words, be less than between the material and the stationary casing, in order for the material to be displaced in a direction towards the discharge end.
It will readily be perceived that, in a vertical or steeply sloping conveyor which is composed of a casing surrounding threads provided with a shaft, the forces with which the material are pressed against the casing will generally be less than the forces with which the material adheres to the threads and the centre shaft. If no special measures are implemented to compensate for this situation, this will have as a result that the frictional forces between the conveyed material and the vertical casing will be less than the frictional forces between the material and the rotating devices. In order to create the conditions for conveying material towards the discharge end, the friction between the material and the casing must consequently be increased. According to prior art technology (a spiral provided with shaft), this is achieved by selecting a high speed of rotation for the screw and, with the aid of centrifugal force, throwing the material against the casing of the conveyor. As a result, these vertical screw conveyors operate, as has already been mentioned, at high speeds, which entails disadvantages such as high power consumption and severe wear, as well as low filling and/or efficiency degree. The low level of filling results often in the situation that only 15-20 percent of the volume of the casing is used, which entails a poor degree of efficiency and also uneven upward conveyance (for example lumping together of sludge). This causes extreme vibrations which result in noise, damage to the conveyor and its anchorage points.
A conventional screw conveyor has a threaded centre shaft. The total surfaces of the threads and the shaft together with the channel-like design of the space between the threads, results in the material readily adhering to the screw and rotating with it, which emplies that no displacement of material takes place in a direction towards the discharge end of the conveyor. There will be an increase of the frictional forces between the material and the screw as a result of the centre shaft of the rotating screw, the shaft also rendering impossible the conveyance of material which may be wrapped round the shaft and, moreover, considerably restricting the possibilities of conveying large pieces of material. A shaftless spiral has a considerably smaller total surface exposed to the material which is to be conveyed.
In a vertical conveyor, the spiral does not normally abut against the casing, while in horizontal and inclining conveyors, the force of gravity results in the spiral coming into abutment against the casing. As a result, in a vertical conveyor there will normally be very slight wear on the spiral and the casing. Moreover, the conveyed material acts as a lining. A certain degree of movement is possible between the spirals, which, however, depends upon the nature of the material conveyed. In addition, different distances are required between casing and spiral, depending upon the material conveyed. As example, it might be mentioned that sludge requires a free play of approx. 5 mm between the spiral and casing, while other materials may require approx. 25 mm.
The present invention relates to a device which satisfies the. above-outlined requirements and obviates the above described drawbacks. According to the present invention, this is attained employing a device according to the characterizing clause of the appended independent claim.
Further expedient and important embodiments of the present invention are disclosed in the appended subclaims.